Management of chronic open-angle glaucoma

The objective of glaucoma treatment A person with open-angle glaucoma is at risk of irreversible blindness. The objective of treatment is to minimise this risk, usually by lowering the intraocular pressure (IOP) so that an individual upper threshold IOP (also known as their target IOP) is not exceeded. However, we must weigh the expected long-term benefit of preserving vision against side-effects, complications, and the long-term cost of treatment – all of which can affect quality of life and the person’s livelihood. Choosing a therapy plan An individual therapy plan is based on a detailed history, visual acuity, and examination of general and glaucoma-related structural and functional details and any changes in these (visual field, disc damage likelihood scale, etc.).

If the rate of progression is low, monitoring can continue, either by only observing the eye or continuing with the same treatment (Group 1). If this was the patient's first assessment or if there is not enough information from previous examinations available, the risk for progression can be estimated (Group 2). An increased risk of glaucoma progression to visual loss is associated with advanced disease on presentation, high intraocular pressure, older age, certain ethnic groups, disc haemorrhages, and thin central corneal thickness, among others. 1,2 If there is an estimated high risk of progression (Group 2), or if there is actual evidence of a high rate of progression (Group 3), an escalation of treatment is indicated. However, it is important to review the current treatment before escalating the therapy;

Laser treatment
Laser treatment can decrease aqueous production by partial destruction of the ciliary body epithelium, which produces aqueous (Table 2a) or by increasing aqueous outflow through the trabecular meshwork (Table 2b). Endoscopic cyclophotocoagulation Similar to TSCPC, with a better complications profile, but more invasive.

Micropulse transscleral cyclophotocoagulation (MP-TSCPC)
Diode laser (810 nm) with short bursts instead of continuous delivery of laser energy to reduce destruction of adjacent non-ciliary tissue. Might also enhance uveoscleral outflow. 8

Table 2b
Laser treatment to enhance aqueous outflow

Laser Comments
Argon laser trabeculoplasty (ALT) Initial treatment with argon laser trabeculoplasty was at least as efficacious as initial treatment with topical medication (GLT). Risk of scarring of the trabecular meshwork and peripheral anterior synechiae formation.
Possibly similar efficacy as SLT e.g., first checking whether the patient was able to purchase the prescribed eye drops and whether they have actually been taking the treatment.

Lowering IOP
Lowering IOP prevents or delays the onset or progression of glaucoma. However, there is no specific IOP threshold, formula, or percentage reduction which applies to all patients. Instead, it is recommended to set and subsequently adapt an individual target IOP. This can be defined as the IOP that slows down the rate of progression of the glaucomatous damage enough to maintain the patient's quality of life and livelihood during their lifetime. 1,3 This definition contains three elements which need to be considered: • Intraocular pressure • Rate of progression of the glaucomatous damage • Quality of life and livelihood.
Analysis of the advanced glaucoma intervention study (AGIS) showed that participants with IOP <18 mmHg at 100% of visits showed no visual field progression. 4,5 However, high-quality prospective data comparing different target IOP levels are not currently available; as such, the trade-off between risks and benefits associated with different thresholds is unclear. 1 Target pressure should therefore be individualised and may need adjustment over time. 1 A single measurement of a high intraocular pressure alone should not trigger a change of management and needs to be put into the context of the other examination results and the history, including self-reported adherence. IOP may also fluctuate within hours or days so that several measurements might provide a better picture of the general level of IOP in an eye. Sometimes a repeat examination on the same day or a repeat follow-up visit within a few weeks might be helpful to decide on the next step, e.g., an escalation of treatment. This also depends on the level of urgency, which can be high for eyes with severe visual field loss and a high rate of progression.
There are several treatment options available to reduce IOP. They can be divided in three groups: medical treatment (usually eye drops but may include oral or intravenous medication, e.g., acetazolamide), laser and surgery. Current cost-effective examples are timolol eye drops, selective laser trabeculoplasty, and trabeculectomy. Other eye drops are only available at considerably higher cost and may not be affordable for some patients in an LMIC context. 6 Some examples are given below, but these will vary depending on the local or regional glaucoma care system.

Medical treatment
Medication (a conservative treatment) can reduce IOP by decreasing aqueous production (Table 1a) or enhancing aqueous outflow (Table 1b). Osmotic agents are not mentioned as they are not for long-term use.

Surgery
There are several surgical options to reduce intraocular pressure, including a selection of minimally invasive options. 'Ab externo' refers to a surgical approach from outside the eye, often involving a conjunctival dissection and scleral incision. 'Ab interno' refers to a surgical approach from inside the eye, usually through the anterior chamber, with a corneal incision.
There are three main categories of glaucoma surgery, each with a different purpose: 1 To enhance aqueous outflow into the sub-Tenon space 2 To enhance aqueous outflow through the trabecular meshwork 3 To enhance aqueous outflow through the suprachoroidal space.

Surgery to enhance aqueous outflow into the sub-Tenon space
Ab externo approach: • Trabeculectomy. The gold standard, low-cost procedure to create a guarded fistula between the anterior chamber and sub-Tenon space, requires adherence to follow-up. The Moorfields safer technique (i.e., using releasable sutures), is also suitable in low-resource settings. 11 • Glaucoma drainage devices. Aravind Aurolab drainage implant, Ahmed valve, Baerveldt shunts (250/350), PAUL Glaucoma Implant.

• PreserFlo
Microshunt. An aqueous shunt between the anterior chamber and sub-Tenon's space; drains more posteriorly.
Ab interno approach: • XEN gel stent. A 6 mm porcine-derived gelatin tube with an inner lumen of 45 µm and outer diameter of 150 µm.

Surgery to enhance aqueous outflow through the trabecular meshwork
Ab externo approach: • Canaloplasty. Dilation of Schlemm's canal using viscoelastics and a suture.
• Trabeculotomy. Accessing Schlemm's canal via a partial scleral flap. A curved probe (trabeculotome) is rotated gently into the anterior chamber to incise through the trabecular meshwork.
• Iridectomy. Improving aqueous flow from the posterior to the anterior chamber.
Ab interno approach:

Surgery to enhance aqueous outflow through the suprachoroidal space
These include: • STARflo, Gold Micro Shunt. Implants to access the suprachoroidal space (an ab externo approach).
• iStent supra. A 4-mm long curved stent with a lumen of 0.165 mm inserted into the suprachoroidal space (an ab interno approach).

Evidence for selective laser trabeculoplasty
The LiGHT trial in the UK 10 showed that selective laser trabeculoplasty (SLT) as first-line treatment of ocular hypertension and primary open-angle glaucoma was safe, cost-effective and resulted in the same quality of life (after 3 years) compared to eye drops.
The Kilimanjaro Glaucoma Intervention Programme (KiGIP) SLT trial compared SLT and Timolol eye drops (with standardised counselling) in patients with moderate and advanced glaucoma in Tanzania. 11 After one year, SLT treatment was successful in 60.7% of eyes, and Timolol eye drops were successful in 31.3% of eyes. In the SLT group, approximately one third of eyes required one repeat session of SLT; in the Timolol group, a similar proportion needed one repeat session of counselling. Safety, acceptance, vision-related quality of life, and preservation of visual acuity were comparable in both groups after one year. Eye care units in the region using a not-for-profit eye care service model would need to treat around 500 eyes per year with SLT to cover the cost of the procedure, charging an amount similar to one year's supply of timolol eye drops.

Focus on the patient, not just the eye
The variety of treatment options available makes it much easier to find an approach to suit the individual patient. There are many factors to consider, including those related to the individual and the health system; see the online version of this article (bit.ly/CEHJpoag) for more detail. It is just as important to ensure that people with glaucoma receive counselling to support their compliance with treatment and quality of life, and to refer them for low vision services and rehabilitation as needed -see the rest of this issue for more information.